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Abstract

Weak momentum dissipation in ultra-clean metals gives rise to novel non-Ohmic current flow, including ballistic and hydrodynamic regimes. Recently, hydrodynamic flow has attracted intense interest because it presents a valuable window into the electronic correlations and the longest lived collective modes of quantum materials. However, diagnosing viscous flow is difficult as the macroscopic observables of ballistic and hydrodynamic transport such as the average current distribution can be deceptively similar, even if their respective microscopics deviate notably. Based on kinetic Boltzmann theory, here we propose to address this issue via the transverse channel voltage at zero magnetic field, which can efficiently detect hydrodynamic flow in a number of materials. To this end, we show that the transverse voltage is sensitive to the interplay between anisotropic fermiology and boundary scattering, resulting in a non-trivial behavior in narrow channels along crystalline low-symmetry directions. We discuss several materials where the channel-size dependent stress of the quantum fluid leads to a characteristic sign change of the transverse voltage as a new hallmark of the cross-over from the ballistic to the hydrodynamic regime.